Adipose tissue development, metabolism and endocrine function are essential for the efficient storage of lipids in adipocytes and for normal systemic metabolism. Obesity alters adipose tissue function and impairs the efficient storage of triglycerides in adipocytes, leading to ectopic deposition of lipids and metabolic impairment in non-adipose tissues. Adipose tissue macrophages through their inflammatory function have been implicated in the development of obesity-induced adipocyte dysfunction and complications, including type 2 diabetes, non-alcoholic fatty liver disease and dyslipidemia. Studies funded by this grant have revealed that obesity leads to the accumulation of macrophages in adipose tissue of obese rodents and humans, that adipose tissue macrophages contribute substantially to local and systemic obesity-induced inflammation and that the macrophage content of adipose tissue is tightly correlated with metabolic derangements associated obesity. Since our original observations most studies of adipose tissue macrophages (ATMs) and other adipose tissue immune cells have focused on the inflammatory phenotype and function of these cells. Indeed studies published since the last application and supported by this grant have found that pharmacological or genetic reduction in adipose tissue macrophages reduces adipose tissue inflammation and improves systemic insulin sensitivity. These and other observations have suggested a model in which ATM inflammatory recruitment and function play a pivotal role in impairment of adipocyte function and in obesity-induced complications. However, the ability of non-inflammatory signals and functions of macrophages to modulate adipose tissue development and metabolism have been less well characterized Work from our laboratory has revealed that among their non-inflammatory functions ATMs respond to and modulate lipid fluxes. We have found that ATMs accumulate in response to lipolysis, activate a program of lipid uptake, storage and metabolism in obesity, and can acutely modulate adipose tissue lipid storage. We hypothesize that ATMs are a developmental distinct population of macrophages that play a role in lipid trafficking through a multistep process of recruitment, differentiation and paracrine action on adipocytes. In obesity, the adaptive functions of ATMs are overwhelmed. The specific aims of this proposal are design to test whether in obesity ATMs accumulate in response to changes in lipid fluxes and identify cellular functions and secretory products that are important in adipose tissue development and efficient lipid storage. Achieving the goals of this proposal will identify critical processes and molecules required for ATM accumulation and function. Success also will provide important insights into adipose tissue physiology, and by identifying the molecular mechanisms that regulate macrophage accumulation and function in adipose tissue, identify potential novel strategies to reduce obesity-induced adipocyte dysfunction and its attendant complications.